Automatic guidance and landing system for aircraft



@et 3, 3.96? G. @LAM 3,345,017

AUTOMATIC GUIDANCE AND LANDING SYSTEM FOR AIRCRAFT Filed April l2, 1965 l2 Sheets-Sheet l RANGE /NVENTOR @2% E0 R G-E @L A H Arrow/YS G. OLAH Filed April l2, 1965 l2 Sheets-Sheet 2 Lu LU Q LL! C l'w h M \i QJ L@ g ii OUTER MARKER M Q Q Q Q" N u QQ S c L g Q CL Lk I\- 31 fw @L 'q M/DDLE MARKER I i E tv GL/DE SLO/ J5 TRANsM/Tgp NVENTOR G EoR e E o 1 A H ATTUR/VEY G. OLAH AUTOMATIC GUIDANCE AND LANDNG SYSTEM FOR AIRCRAFT Filed April l2, 1965 l2 Sheets-Sheet 5 R m mag@ .,w m, QQ V PGR Q m 2 N mnwmm MQW@ mz n.: i V Eso Qm MQ m m mm2@ QS@ ow muz @Sow @Q Qm @www QQ uw@ 8 n m u@ RNA up N mmv Rm. H mw k www. uw Suwnwlp h @w .1. .p w mm x m mw 111? Im. N k 8 l' n A wzou mw TQ ik I l` W.

G. OLAH ci. 3, i967 AUTOMATIC GUIDANCE AND LANDING SYSTEM FOR AIRCRAFT l2 Sheets-Sheet 4 Filed April 12, 1965 MUEFNQ /N VEN TOR 12 Sheets-Sheet 5 AUTOMATIC GUIDANCE ND LANDING SYSTEM FOR AIRCRAFT @mm Si 8 QQQ @a Filed April l2, 1965 AUTOMATIC GUIDANCE AND LNDNG SYSTEM FOR ARCRAFT Filed April l2, 1965 l2 Sheets-Sheet 6 f4 MONITOR/NG FEEUSACK n SI5/wu.. UTD y "2 32! y UML/55A @HD5 CROSS SIGNAL 5..O/ LOCAL/SEP g/g/VL DIST OUTPUT AZ/MUTH /NTEGRAT/UN .m 4Z/MHT# C9055 ST5/"EM ,y LOCAL/5E@ VELoc/Ty OUTPUT 28/ 0:1055 o? /g fZ-Zi 0cm/SER Momma/NG f m FEEDBACK D/STANCE 605,5 m c9055 ,QA/W35 5M f S/GNAL Hm 3! (SL/DE SLOPE STEER/NG D/STANCE MU/.T/PL/ER "5mg/ALS To ZJ AUTO /LUT J0 M055 ZT@ f @1 /DE SLOpE ELEVAT/ON @figg/Tm 2,@ /NTEGRAT/o/v .-v SYSTEM T CTU/DE SLOPE AVI/6'` VELUc/TY OUTPUT z/ f y E EX N ff f z AXIS u /NmT/AL VERT, RANGE U/v/T ACCELN RgoLUT/Om AND PLATFORM N UNIT RESOLVED @Tx/N CUNTROL yEnc/T@ if l y VELOC,

4 f DIST LOCAL/55p i3 A TR Z0 GUIDE SLOPE rm N5. @UNM/AY f RTWTON HEA D/NG OTFMA RMR f? I GUIDE /w YZ Rif@ -Qaw A, wlgr@ RANGE 5L0PE ,4A/5L@ MM- A/RFTELD DMA OUTER MIDDLE MARKE/2 MARKE/2 T F7 G' 6 "VPU UN/T s/GNAL s/GNAL INVENTOP GEORGE ULAH @et 3, 967 G. OLAH 3,345,017

AUTOMATIC GUIDANCE AND LANDING SYSTEM FOR AIRCRAFT Filed April 12, 1965 12 sheets-sheet v ACCEL ERMEVER HEAD/NG CON TPOL G YRO /N VE N TUR GEORG-E OELA H @et 3:, W67 G. @LAM 3,345,017

AUTOMATIC GUIDANCE AND LANDING SYSTEM Foa AIRCRAFT Filed April l2, 1965 l2 Sheets-Sheet 8 QUA/WAV HEA D/NG f "Aga AA QJ A Q5 COMPASS E/moR m /r X ACCELERQMETER Ax/s A ,ff/C. 9.

Ru/WAY gfx de Y Y@ y@ I A COMPASS ERROR T`=IR A/RFRAME OR/C/NAL Pos/'HON Ax/s A, j\ 0F ACCELEROMETCR TRACK FIG/L /NVENToR GEORGE oLAH ,QTToR/VEYS G. OLAH AUTOMATIC GUIDANCE AND LANDING SYSTEM FOR ARGRAFT Filed April l2, 1965 G. 5, BEAM CENT/QE mls. mAfvM/TTEAS l2 Sheets-Sheet 9 F/G. l0.

GE'dRe-E @LAM IN VEA/TOR ATTORNEYS AUTOMATIC GUIDANCE AND LANDNG SYSTEM FOR AIRCRAFT Filed April 12, 1965 G. QLAH l2 Sheets-Sheet 1 0 Get. 3, i967 INVENTOR TTOPN YS GEORGE @AAH BY y ct. 3, Egg? G. OLAH 3,3457

AUTOMATIC GUIDANCE AND LANDING SYSTEM FOR AIRCRAWIv Filed April l2, 1965 l2 Sheets--Sheel 1l GEORGE a/ AH mfg/WIKE16 TTURNEYS ct. 3, 96'7 G. @L AH 3,345,017

AUTOMATIC GUIDANCE AND LANDING SYSTEM FOR AIRCRAFT Filed April l2, 1965 12 Sheets-Sheet l2 A @c r n FROM PL A TFORM (mw ,4x/5) /53 ,-i Y /40 n /54 H-QN y FROM /NTEGRA Top5 MODULA To@ CORRECT/ON T0 YAw SERV@ INVENTO? G EORGE 0L. A H

BY l

www

ATTORNEYS United States Patent O AUTOMATIC GUIDANCE AND LANDING SYSTEM FOR AIRCRAFT George Olah, London, England, assignor to Elliott Brothers (London) Limited, London, England, a British company Filed Apr. 12, 1965, Ser. No. 447,380 Claims priority, application Great Britain, Apr. 14, 1964,

15,469/ 64 12 Claims. (Cl. 244-77) The present invention relates to an all-Weather aircraft landing system and particularly to such a system which is intended to relieve the pilot of action during final approach, flare-out, and (if required) after touch down. The invention relates to cases in which it is -assumed that I.L.S. facilities exist, or some system which affords to the aircraft indications or signals of the same nature Ias those afforded by I.L,S. and the term I.L.S. is intended to include any such facilities.

A primary requirement for an all-weather landing system is to provide during the landing approach up to touchdown reliable information on deviations from desired glide slope path and the velocities of these deviations independent of external disturbances (gusts, windshear or as the latter is sometimes called wind gradient). These deviations are herein referred to as cross beam deviation and are measured in the case of azimuth or lateral deviations in horizontal direction perpendicular to the flight path and in the case of elevation deviation in Avertical direction. I Y

I.L.S. alone does not fulfill this requirement.

(a) It leaves an information gap between inner I.L.S. limit and touch-down in azimuth, `and between inner I.L.S. limit 'and radio altimeter altitude in elevation.

(b) It is very noisy, especially on cross beam velocity; efficient filtering would cause a large time delay in the detection of external disturbances, preventing timely corrective control action.

An object of the present invention is to provide a system to close the information gap and to achieve optimum filtering of I.L.S. information without incurring an intolerable time lag in detecting external disturbances by using I.L.S. information to monitor and to introduce corrections to, an inertial navigation system.

It is also an object of the present invention to provide such a system which can be operated as a pure inertial system during run-out after touch down.

The present invention provides an aircraft landing system comprising an inertial navigation system to produce cross-track lateral and vertical distance information signals, means for receiving and converting I.L.S. angular information to lateral and vertical distance information signals, a comparator circuit to compare the former signals with the latter signals and to produce difference signals in response to any difference therebetween, iand means for correcting operation of the inertial system in response and according to said difference signals.

The basic information provided by the inertial system consists of the acceleration components along the north,

east and vertical axes. These -accelerations can then be resolved into the following components:

(a) Azimuth cross beam or lateral acceleration, obtained by resolving the north and east accelerations in a direction perpendicular to runway axis (knowing the runway heading).

(b) Range acceleration, obtained by resolving the north and east accelerations in a direction parallel to the runway.

(c) Elevation cross beam or vertical acceleration, obtained by resolving range acceleration and vertical acceleration in a direction perpendicular to the center of the glide slope beam.

Cross beam velocities and cross lbeam distances can be obtained by integrating these cross beam acceleration components.

A perfect inertial system, carrying out the integration process from take-off to landing, would yield correct cross beam distances and velocities all the time. However, in the time from take-off to landing approach an actual inertial :system develops accumulated errors, which may be relatively small for long range navigational purposes (say of the order of fa few miles distance error and say 5 ft./ sec. for -any particular velocity component) but far in excess of what can -be permitted at landing approach. Consequently the pure inertial information has to be monitored and adjusted from information obtained primarily from I.L.S.

In view of the large noise content of instantaneous I.L.S. indication and the still larger noise content of its derivative, it is practically impossible to readjustY the inertial system instantaneously at the interception of I.L.S. Instead, a gradual adjustment is proposed. I.L.S. distance information is continuously compared during the whole I.L.S. landing approach with the inertial distance outputs, and the differences of the two are fed back ascorrections to the inertial system.

In a further aspect of the invention, therefore, the inertial system employed comprises accelerometer means to produce range acceleration, azimuth cross-track acceleration and elevation cross-track acceleration, and integrating-circuit means for integrating each of the latter two accelerations to provide the lateral and vertical velocities -and the lateral and vertical distances.

The accelerometer means may =be constituted and may derive the required accelerations as follows:

(A) Derivation of accelerations from a complete gyro stabilized platform, which is set up prior to the landing operation. The platform carries 3 accelerometers in, nominally, N, E and vertical, the accelerometer system being set to O output at 1 g. Azimuth cross track and longitudinal acceleration are obtained from the original out-put by resolving them according to the data given by run-way heading and glide slope angle. The accuracies required would be those afforded by a high grade and expensive platform.

(B) Derivation of acceleration components based on attitude information from existing directional gyro and vertical gyro. There are two possibilities:

(a) A slaved platform is provided with its main axes (outwards to inwards) in the 0 position longitudinal, transverse, vertical. The outer gimbal is set according to bank attitude, the middle gimbal to pitch attitude indicated by the vertical gyro, the inner gimbal is set to relative heading (compass indication-run-way heading). The inner gimbal represents a horizontal platform on which the accelerometers are located, the lateral accelerometer being positioned perpendicular to its axis and thereby directly measuring lateral acceleration. Longitudinal and vertical accelerometers are mounted inclined to the glide slope angle and measure also directly without further resolution the respective accelerations. The outputs have again to lbe set so that they are at 1 g vertical acceleration. The horizontal platform may carry an integrating gyro the output of which is fed to the integrating means to correct vertical gyro errors. Alternatively the platform may carry a control gyro to maintain platform attitude.

(b) The accelerometers are mounted in aircraft axes and their outputs resolved according to pitch and bank data from a vertical gyro (VG.) and relative heading from a directional gyro (D.G.). The resolution procedure is very involved, especially as no constant compensation for vertical 1 g can be provided, as in case A and B(a), but the g compensation dependent on actual pitch and bank.

Of these three possible arrangements, the slaved platform is preferable on the basis that it is the simplest and the least expensive.

To compare I.L.S. information with inertial output, the angular I.L.S. information must be converted to distance information by multiplying with range. Range may be obtained, when a complete gyro-stabilized platform is used, by appropriate integration of range acceleration or, when a slaved platform is used, by continuous triangulation from altitude in relation to I.L.S. glide-slope information, and altitude may be obtained by barometric signals. Range may be monitored at the outer and middle markers and initially on interception of the glide-slope beam by the range derived from triangulation.

The invention will now be further described, by way of example only, with reference to particular embodiments as illustrated in the accompanying drawings in which:

FIGURE 1 is a block diagram for an azimuth guidance circuit of one embodiment of t-he I.L.S. monitored inertial system of the present invention;

FIGURE 2 is a diagram of a flight profile in the vertical plane;

FIGURE 3 is a diagram showing the variation of time (integration) constants with range;

FIGURE 4 is a graph showing indicated errors of the embodiment in respect of the Dallas localizer transmitter in terms of azimuth Velocity error, azimuth indicated distance error and I.L.S. distance error with range;

FIGURE 5 is a graph showing indicated errors of the embodiment in respect of the Washington glide slope transmitter in terms of elevation velocity and distance errors and I.L.S. distance error, with range;

FIGURE 6 is a block ydiagram of a complete system incorporating the azimuth guidance circuit of FIGURE 1 and a like circuit for elevation guidance;

FIGURE 7 is a diagram showing an arrangement for slaving an accelerometer platform to a vertical and a directional gyro;

FIGURE 8 is a sketch of an embodiment of a slaved platform as illustrated in FIGURE 7;

FIGURE 9 is a diagram showing acceleration error due to compass error and longitudinal acceleration;

FIGURE 10 is a diagram showing the derivation of range from barometric altitude and glide slope information;

FIGURE 11 is a diagram showing the parameter involved in correction of compass error;

FIGURE l2 is a lblock diagram of the complete system of a second embodiment of the invention;

FIGURE 13 is a block diagram of the circuit employed in the system of FIGURE 12 for computing range and I.L.S. `distance with facilities for monitoring the former at the middle marker; and

FIGURE 14 is a block diagram of a circuit for efecting runway correction which may be employed in the system for FIGURE 12.

Referring now to the drawings, FIGURE 1 shows the azimuth guidance circuit employed in the irst embodiment of the invention. The elevation guidance circuit is substantially the same and the extent to which it may differ is discussed below. In view of this, it is not `considered necessary separately to describe and illustrate the elevation guidance circuit. It may be seen from FIGURE 1 that the circuit comprises two integrating circuits 1, 2 (hereinafter called integrators) and a differential amplitier 3 with integrator 1 receiving the output of an azimuth accelerometer from a complete gyro-stabilized platform 4, integrator 2 the output of integrator 1 viz: azimuth velocity, and t-he differential amplifier receiving the output of the integrator 2 viz azimuth deviation, and the output of a multiplier 5 in which I.L.S. angular information supplied by the I.L.S. localizer receiver 19 is multiplied by range. The output of the differential amplifier is fed back respectively to the accelerometer output, the input of integrator 1 and the input of integrator 2, by feed-back paths 6, 7 and 8; feed-back path 8 containing a further integrating circuit 9 (hereinafter called integrator 9) to integrate the feed-back signal. Each of the feedback paths contains circuit-breaking means 10 and a variable gain unit 11 to determine the time constants of integrators 1, 2 and 9. The integrating circuits presently preferred are of the type known in the art as Miller, or the type known as Velodyne. The outputs taken from the circuit are corrected lateral acceleration indicated by "tjt, oorrected lateral velocity indicated by 1]* and corrected lateral deviation indicated by yt.

The basic properties of the system may be derived ls Laplace Transforms and can be summarized as folows:

(1)(a) During the I.L.S. monitored period the system gives cross 'beam distances and velocities corresponding to ltering the I.L.S. distance with a first order iilter with time constant of the order of 10 to 20 secs., and of I.L.S. velocity by a second order filter with the same order of time constants. The response to external disturbances -due to gusts, etc. (which in a pure I.L.S. system would have time lags corresponding to t-he filter time constants) would be practically instantaneous. Thus'cross beam distances and velocities obtained from the system would be eminently suitable for controlling the aircraft. The control loop would be essentially independent of the I.L.S. filtering lag, thus enabling a tight control to be exercised. At the same time the high degree of filtering would provide very smooth steering signals.

(b) At the inner I.L.S. limit the correcting feed-backs are switched olf and the system is working on pure inertial information, adjusted by previous I.L.S. monitoring. It continues to give cross beam distance and velocity indication until touch down or even on the runway in azimuth and up to the are limit (where radio altimeter takes over) in elevation. The system indication during -the pure inertia phase is again essentially independent of external disturbances.

(c) In addition, range information is provided up to the end of the run out.

(2) Error anaZyss.-Errors can arise from the following sources: y

(2.1) I.L.S. eNom-These are substantially reduced by smoothing, thus enabling the I.L.S. range to be extended even at facilities which 'become very noisy beyond the 200 ft. altitude limit.

(2.2) Initial setting errors- These cause decaying transient indication errors, which `are as follows:

(a) Initial distance error, Yo.-lf no other provisions are made at the start of I.L.S. monitoring the aircraft may be anywhere within the beam, so that if the second integrator starts with zerov output, the maximum known distance error would be that corresponding to 'half the beam width. This initial error can be substantially reduced by increasing the gain for a short time, i.e. reducing time constant T1 of the feed-back to the second integrator to, say the order of 0.1 sec. Then within a few tenths of a second the initial inertial distance indication becomes almost equal to the I.L.S. distance indication and the initial distance setting error becomes equal to the I.L.S. distance error. This is very much smaller and will not exceed one fifth of therhalf beam width. The corresponding indication errors can be included in the evaluation of I.L.S. errors.

(b) Initial velocity error, Y0.This is the laccumulated velocity error of the inertial navigator from take off to the landing approach. It may amount to 5 'ft/sec. maximum (95% probability).

(c) Initial acceleration error resulting from vertical and accelerometer bias error, Y0.- This can be expressed as a resultant acceleration error. It is assumed that it corresponds to an equivalent vertical error of one minute of arc, corresponding to an acceleration error of .0l "ft/sec.2 (95 probability). If no third integrator were provided, the effect of the initial accelerometer error would lne-apart from smaller transientsa steady state error, which may become comparatively large with the large time -constant required to smooth I.L.S. Representative values in azimuth would be l1 ft. at touch-down and 30 ft. at the end of the run out.

The third integrator substantially reduces the steady state error. In elevation the effect of vertical error is reduced by the sine of the glide slope angle and becomes very small, thus the resulting acceleration error can be assumed to be 1U4 g, representing accelerometer bias error only. Due to' the shorter duration of the pure inertia phase its effect is further reduced. Representative values at the 'beginning of flare out would be a distance error of 2.4 ft. and a velocity error of 0.13 -ft./sec.

This error may be tolerable; thus in elevation the third integrator can possibly be omitted.

(2.3) Platform azimuth error.-A typical value of platform `azimuth error \p=0.25 (95% probabilgust effects; during the run out there is a considerable deceleration (say of 2.0 g). At touch down the distance error due to platform position and range acceleration error is small (onder of 1 ft. or less)-at the end of the run out it may amount to 16 ft. at a ground speed of 220 ft./sec. and 4.8 ft. at 120 ft./sec. (2.4) Instrument errors.-'I`hese are:

(a) Vertical gyro Wander rate and corresponding change of platform position (assumed to be 0.02/hour).

(b) Change of accelerometer bias.

(c) Drift of third integrator. All these errors Vgive the same effect-a resulting change of acceleration input bias during the I.L.S. monitored and subsequent -pure inertial phase. Error components (2.4) (a), (b), (c), can be represented as an initial setting error Yo, representing acceleration error rate. Its total R.M.S. value can be estimated as that resulting from the gyro wander rate only with a 50% addition. The steady state value is not eliminated by the third integrator.

(d) Errors of the rst and second integrators,

differential amplifiers and resolvers are generally so small compared with other instrument errors that they can be ignored, when computing resultant R.M.S. errors.

(2.5) Range errors- Range indication obtained by integrating longitudinal velocity is set and respectively reset at the following points:

(a) Glide slope interception. It may be seen from FIGURE 2 that the glide slope range is sin (17 -K the localizer range is Ry Ryz-I-RI (Ryz=distance between localizer and glide slope transmitters). (b) At the center of the outer marker beam. (c) At the center of the middle marker beam.

Adjustments (b) and (c) could be achieved in the following way:

On reaching theedge of the marker beam, the range is reset according to the marker range and the speed obtained from velocity integration is reduced by half, and when leaving the inner edge of the marker beam, the speed of integration is reset to its original value.

Assuming an error of h= ft., and I.L.S. error=1/s of K, and a longitudinal velocity error of 4 ft./sec., an error of outer marker center indication of 25()v ft., and an lerror of middle marker center indication of 30 ft., the

maximum values for absolute range errorsV (6R) and relative range errors R/R are tabulated in Table 2.5.1.

TABLE 2.5.1. RANGE ERRORS Interception Outer to Middle 50 ft. alti- Ground to Outer Middle Marker to tude to end speed, Marker Marker 50 ft. altitude of runout itz/sec Glide Slope R., It 40, OOO-27, G00 27, GOO-4,000 4, OOO-1, 000 Localizer:

Ry, 50, (10o-37, U00 37, 00C-14, 000 14, OOO-11, 000 6R, ft 4, OOO-4, 400 250-1, 020 30-130 130-270 120 Glide Slope 5R/R5, percent 10-16 9-25 1-13 Localizar 5R/Ry, percent 8-12 l-7 0-1 ity). This has no influence as long as the ground speed is constant. However, if there is an acceleration along the range, a cross beam acceleration error of il/V occurs Iin azimuth, causing cross beam distance and velocity errors at touch down and at the end of the run out. Range accelerations before The effect of range errors on cross beam indication errors is twofold, Firstly, I.L.S. distance error can be increased (or decreased) approximately by the percentage R/R. Secondly, in the absence of range errors the system response to external disturbances (apart from negligible instrument lags) is instantaneous, i.e. if the touch down may occur due to windshear and steering signals are utilized in the aircraft control loop,

the feedback through the I.L.S. monitored inertial system does not involve any phase lag or frequency variable gain. This is slightly modified when range errors are present. In the Worst condition (R/R=%) the I.L.S. monitored inertial system may give an increase in gain of 2 db and a phase lag of 9. The effect is comparatively small and proportionately less for smaller values of R/ R. Furthermore, for frequencies above 0.08 c./s., phase and gain changes reduce to negligible values. Thus range errors will effect control stability only very slightly.

(3) Choice of time constants-The following considerations have been applied:

(a) The system should be critically damped to avoid resonance effects.

(b) Feedback through the third integrator generally increases indication errors due to I.L.S. Thus this feedback should be just large enough (or T3 small enough) to wash out the initial acceleration error sufiiciently. These considerations determine the ratio of time constants T1, T2, T3 during the latter part of the landing approach. Given the ratios, the system performance can be characterized by a single time constant T.

(c) The actual value of the time constant T is also subject to considerations (a) and (b). A larger lter time constant reduces the errors due to I.L.S., but reduces the efficiency of the initial acceleration error washout. A best compromise must thus be sought.

(d) A sufiicient washout of the initial velocity setting error has to be provided. It has been found that a constant gain system with time constants according to requirements (a) to (c) Will give an excessive indication error due to errors in Y (e) Initial distance error Yo has to be reduced to the I.L.S. distance error value (see section 2.2(a)

(4) Variable gain system- The conflicting requirements above can best be met by a variable gain system. The subsequent evaluation was based on a variable gain employed with the following characteristics during an approach of four successive phases as shown in FIG- URE 3:

Phase 1.-Duration approximately l sec. at a groundspeed of 220 ft./sec. and T1=0.1 sec. IFeedbacks to first and third integrator disconnected. Initial setting error reduced to I.L.S. distance error value.

Phase .2. Duration approximately 43 sec. at

groundspeed 220 ft./sec. Feedback to third integrator stil disconnected; time constant winds up proportionally with time from T=1.25 sec. to T=20 sec. This phase serves to reduce the initial velocity setting error to approximately 1/20 of its original value; the third integrator is omitted to avoid an accumulation of large unwanted acceleration corrections.

Phase 3.-Frorn here on up to I.L.S. limit. Feedbacks to all integrators connected; time constant T=20 sec. at 220 ft./sec. groundspeed.

Phase 4.Pure inertia phase. Feedbacks to all integrators disconnected.

FIGURE 3 shows the variation of time constants with range. Furthermore, best performance is obtained by making time constants (T1, T2, T3) inversely proportional to groundspeed. In a system of this kind the error in cross beam distance indication due to I.L.S. errors becomes independent of groundspeed. The error in velocity indication becomes proportional to groundspeed, and the error in acceleration indication becomes proportional to the square of groundspeed at any point in the I.L.S. monitored landing approach.

The indication errors in distance velocity and acceleration at the I.L.S. limit caused by I.L.S. errors uniquely determine (apart from random initial setting and instrument errors) the performance in the subsequent pure inertial phase; i.e. errors at touch down, flare limit and at the end of the run out. These parameters represent the resulting effect of all bends in the I.L.S. beam; and can thus be considered as figures of merit for a particular runway. Generally the influence of the acceleration error term is small and can be ignored.

(5) Evaluation of I.L.S. errors-Evaluation is based on data published in R.T.C.A. paper 31-63/DO-118 for angular I.L.S. noise for a number of airfield facilities. The indication errors due to I.L.S. were evaluated on a digital computer using transfer functions for indication errors. The initial distance error is also included. FIG- URES 4 and 5 show the evaluation for the Dallas localizer facility and the Washington glide slope facility. The I.L.S. distance errors are also shown. The high frequency noise components (above 2 c./s.) were ignored as their effect is negligible due to the large filter time constants. The dotted part of the curve was derived by assuming that the I.L.S. angular error changes linearly with the average slope over the range given by data from the R.T.C.A. paper. As at inner I.L.S. limit, the indication errors are mainly determined by I.L.S. errors in the neighborhood of that limit, and I.L.S. errors beyond 20,000 feet glide slope range have only a very small influence, deviations of actual I.L.S. errors from the assumed linear approximation will cause only insignificant errors at the I.L.S. limit.

Tables 5.1 and 5.2 give computed indication errors ET, ET at touch down and Es, ES at the end of the run out for the localizer; and EF, EF at the beginning of flare out (50 ft. altitude) for the glide slope facilities. The inner I.L.S. limit is taken at 50 ft. altitude and also at 100 ft. altitude.

TABLE 5.1. COMPUTED INDICATION ERRORS FOR 50 FT. I.L.S. LIMIT l l l Touch Down Distance Error Touch Down Velocity Error Run Out Distance Error Run Out Velocity Error Localizar Facilities Er, ft. ET, it. sec. Es, It. Es, ft. sec.

V ft./sec 220 120 220 170 120 220 170 120 220 170 120 Dallas 5. 2 5. 2 5. 2 803 620 438 23. 0 11.3 3. 2 Ontario: 17. 8 17. 8 17.8 160 123 087 7. 9 12. 7 15. 8 S.Franc1sco 20.0 20.0 20. 0 078 037 026 20.7 20. (i 20. 4 007 009 .017 S. Lou1s.- 13.1 13. 1 13.1 050 .039 027 14. 3 13.9 13. 9 016 022 022 Los Angeles. 26. 7 26. 7 26. 7 120 .092 066 20. 9 23. 7 25. 4 223 440 .083 Minneapolis. 3.3 3.3 3.3 552 .427 .302 24.8 15.8 9. 2 .720 506 .329 Duluth 5.9 5.9 5. 9 265 205 145 4. 2 0 3.1 .334 238 156 Glide Slope Flare Limit Distance Error Flare Limit Velocity Error Facilities EF, ft. EF, ft./sec.

Washington. 5 5 5 374 290 204 New Orleans 5 5 5 .198 .153 099 Chicago 2 2 2 246 190 134 TABLE 5.2. COMPUTED INDICATION ERRORS FOR 100 FT. I.L.S. LIMIT Loealizer Facilities ET, ft. ET, fin/sec. Eg, It. Eg, ttf/see.

V tJSec 220 170 120 220 170 120 220 170 120 220 170 120 Dallas 3. 9 3. 9 3. 9 973 728 398 37. 4 23. 9 11. 4 1. 035 771 403 Ontario. 21. 6 21. 6 21. 6 073 057 040 15. 1 18. 6 20. 5 310 168 078 S Francisco- 23. l 23. 1 23. 1 128 O99 070 26. 9 25. 6 29. 3 096 084 065 ouis. 6. 2 6. 2 6. 2 g. 134 l. 104 073 4 2. 9 4. 7 214 142 086 Los Angeles- 12. 8 12. 8 12. 8 470 364 256 6. 3 1. 9 7. 8 659 451 289 Minneaplis 1. 5 1. 5 1. 5 468 362 255 19. 7 .12. 0 6. 5 609 429 278 u luth 3. 3 3. 3 3. 3 308 238 168 8. 5 3. 5 I 386 275 181 Gllgtlge EF, fr EF, msec.

Washington 1.5 1.5 1.5 .363 .281 198 New Orleans 1. 3 1. 3 l. 3 212 164 lll Chicago 2. 0 2. 0 2. 0 260 200 140 For computation of indication errors at the end of the 2 run out a deceleration of 0.2 g was assumed.

(6) Reducing errors by I.L.S. calibration-Ignoring random errors and acceleration errors at the I.L.S. limit and referring to section 3, the indication error Es at the end of the run out can be computed from the I.L.S. limit values ET and the velocity error at 220 ft./sec. ground speed ETW, as

ETzzoxv where ts is the time from I.L.S. inner limit to the end of run out. ES can be measured as the difference of indicated position and actual position at the end of run out, v at I.L.S. limit is given by the inertial equipment and can be recorded and ts can be measured. Ignoring all other errors, ET, TZZO could be obtained from two preliminary landings under good visibility conditions. In View of the other errors a larger number of preliminary landings (say of the order of 10) would be required and ET, T220 be determined -by the method of least squares. It then ET is applied as an open loop correction during the whole I.L.S. approach to the distance output and as correction to the output of the first integrator, indication errors at touch-down and the end of the run out may be reduced to a fraction of their original values.

This procedure may be useful for localizer correction with the I.L.S.limit at ft. altitude (decision limit); at 50 ft. altitude I.L.S. limit or below the errors may be tolerable without `any correction. The glide slope errors are in any case very small, and require no correction.

(7) Other errors-The effects of initial setting errors and instrument errors or indication errors are given in Tables 7.1 and 7.2; the values yof the individual errors are those given in section 2. They represent 95% proba- Ibility values.

TABLE 7.1 INITIAL SETTING ERRORS FOR 50 FT. I.L.S. LIMIT Localizer Facilities VE'r, it. T, ftJseo. Es,it. gLiL/sec.

V fla/sec 220 120 220 170 120 220 170 120 220 170 120 if.; 1. 9 3. 1 6. 2 Y 113 146 206 7. 0 7. 7 10. 5 192 208 255 Platform Position. small small small small small small 16. 2 9. 7 4. 8 960 758 524 Total RMS 2. 0 3.3 6.6 12 16 22 17.9 12.8 12.3 .98 .79 .59

Glide Slope Facilities Er, ft. F, ftJsee.

if.. .3 .4 .e Y .031 .031 .031

Y; small small small small small small Platform Position small small small small small small Total RMS .4 .6 1.2 .04 .05 .06

TABLE 7.2 INITIAL SETTIN G ERRORS FOR 100 FT. I.L.S. LIMIT Locallzer Facilities ET, ft. T, ft./sec. Es, ft. Es, ft./see.

V ft./Sec 220 170 120 220 170 120 220 170 120 220 170 120 "flo 2. 4 4. 0 8. 0 141 176 260 8. 2 10. 2 14.2 237 280 336 if.' .4 .9 2.7 .025 .042 .084 1.7 2.8 4.5 .044 .07s .104

Platform Position. y small small small small small small 16. 2 9. 7 4. 8 960 758 524 Total RMS 2. 6 4. 2 8.5 .15 19 28 18. 5 14. 3 15. 8 1.00 80 63 Gude slope Facilities Er, n. r, rit/sec.

ir.. .5 .6 .s .035 .035 .035

irl, .5 .s 1. .032 .042 .059

small small small small small small Platform Position small small small small small small Total RMS 7 1.0 1. 7 50 06 07 (8) Conclusions from computed resuIts.-Examples 0f apparently poor quality facilities (except Minneapolis and Duluth) were chosen from the R.T.C.A. paper for the purpose of I.L.S. evaluation and no attempt was made to optimize for any individual facility. The results indicate that satisfactory performance can be expected for glide slope and for localizer to touch down limit in azimuth. Even at the end of the run out, the indication errors due to I.L.S. errors are reasonable. However, in view of the comparatively large errors due to platform azimuth error during the run out period it could be feasible to adopt the calibration procedure as described in section 6. Furthermore, the results quoted may not represent the absolute optimum achievable, which could be obtained by a closed investigation of the optimization of the variable gain system.

Possible improvements could also be achieved by extending the localizer limit further inwards, which seems to be feasible even for very noisy facilities.

(9) lnstrumentiaton.-The functional block diagram of the azimuth guidance and elevation guidance system is shown in FIG. 6. Where appropriate, the 'reference characters of FIG. l have been incorporated in FIG. 6 but for the sake of simplicity, the various error correction signals of FIG. 1 have not been noted in FIG. 6. In FIG. 6, all of the outputs of the inertial platform 4 are shown applied to the axis resolution unit 20. The north and east acceleration outputs of the inertial platform 4 are resolved, `knowing the runway heading 50B, in a horizontal direction perpendicular to the runway axis to provide the azimuth cross beam acceleration output The north, east and vertical acceleration outputs are resolved, knowning the runway heading, in a vertical direction perpendicular to the runway axis to provide the elevation cross beam acceleration The north and east velocity outputs are resolved, knowing the glide slope 1 into velocity v along the beam. Runway heading and glide slope angle signals are supplied by the airfield data input unit 22 and, as well, this unit provides the various signals shown applied tothe range and gain control unit 24.

The unit 24 derives range x from the integration of the velocity v` along the beam, the integration constants being set at glide slope interception and at the inner and outer markers as described in section 2.5. The gain control signals to the variable gain units 11 (FIG. l) are provided by the output 26 of the unit 24 and are applied to the azimuth and elevation integration systems 28 and 30 according to the description in section 4. The azimuth integration system 28 consists of the several integrators 1, 2 and 9 and their associated variable gains 11 as described in conjunction with FIG. 1 and as shown therein enclosed by the dashed-line designated by the reference character 28. As previously described, the elevation acceleration system 30 may be identical with the system 28 or may exclude the third integrator 9 (see section 2.2(c) In any event, the I.L.S. glide slope signal derived from the receiver 32 is applied to the range multiplier 5 to produce the signal representative of the vertical distance z from the glide slope path, which signal is cornpared at 3 with the corrected elevation cross beam deviation z* to provide the feedback signal to the system 30 comparable to the feedback signal from comparator 3 for the system 28.

The outputs of the comparative 3 and 3' are smoothed at 34 and 36 for the purpose described below in section 10.

The computations involved in the system can be either performed by analogue or digital means, analogue integrators being accurate enough for this purpose. However, digital computation appears to be particularly attractive as it is probably reasonable to assume that an airborne digital computer forms part of the inertial navigational equipment. Airfield data can easily be introduced, e.g. in digital form on a punched card for each airfield runway. The only additional equipment would be analogue/ digital conversion units for the I.L.S. signals-possibly already provided in the computer. Thus additional hardware requirements would be reduced to a bare minimum.

( 10) Self-montorng.-During the I.L.S. monitored phase the smoothed outputs of the differential amplifiers 3 and 3 comparing monitored output and I.L.S. lgives a criterion for the performance of the computer system-any errors indicating a differential output of larger than say l0 ft. would be a failure indication of one of the system components and consequently a switch over to a duplicate equipment could be effected. During the pure inertial phase (never exceeding 50 sec. duration to the end of run out, 8 sec. to the flare limit and 25 sec. to touch down) the above criterion is no longer valid.

A duplicate system comprising two self-monitored inertial equipments with automatic changeover in the event of a failure is thus required to provide failure survival for the complete automatic landing period.

In the abovedescribed embodiment, the basic accelerations required are provided by a completely gyro stabilized platform. A cheaper way, however, of producing the basic accelerations is to employ an accelerometer platform slaved to the aircrafts existing vertical gyro (V.G.) and directional gyro (D.G.), and in a further embodiment of the invention such a platform is employed. The manner of slaving the platform is shown in FIGURE 7 and a construction of a suitable platform in FIGURE 8. Referring to FIGURE 7, the outer gimbal and the middle gimbal are set according to bank attitude and pitch attitude (a) respectively as indicated by the vertical gyro, and the inner gimbal is set according to relative heading (gb) as determined by the directional gyro (Le. compass) indication in relation to runway heading. The inner girnbal represents a horizontal platform 12 on which the 4accelerometers are mounted; the lateral accelerometer measuring directly cross beam or lateral acceleration. In -FIG URE 8 the lateral accelerometer LA. is shown mounted on the horizontal platform 12 together with a floating gyro 13 of medium quality, e.g. with a wander rate of 0.2/hour, to reduce lateral accelerometer error due to Wander of the vertical gyro, to an acceptable limit e.g. 3.12 105 ft./sec.3. The other accelerometers i.e. elevation and range are not shown for the sake of simplicity but are mounted inclined to the -glide slope angle. These accelerometers also measure without further resolution the respective accelerations. The outputs of the three accelerometers need to be set to give a output at l g vertical acceleration.

The use of a slaved platform provides an ecacious and relatively cheap means of producing the basic accelerations but it does give rise to an additional source of error in that the position of the lateral accelerometer is determined by relative heading Which is subject to error in the D G. and this error affects compass indication as shown in FIGURE 9. If there is a compass error from this source, a longitudinal deceleration causes an acceleration error.

Longitudinal accelerations occur during the landing approach due to wind effects, in particular due to windshear and gusts, on the assumption that a constant airspeed is maintained. During the landing approach at constant lairspeed the aircraft generally accelerates as the headwind decreases due to windshear (wind gradient).

During the I.L.S. monitored period the acceleration error is reduced due to the feed back system. If the Windshear be linear, its effect Would be practically fully corrected by the action of the proposed third integrator; in all other cases (nonlinear windshear, gusts) the correcting action of the feedback is only partial; als'o acceleration errors occur in the subsequent pure inertial phase after I.L.S. has ceased to be effective.

The effect of wind variations, particularly gusts, is somewhat mitigated by aircraft inertia; in the following considerations it is assumed that the essential aircraft response has a' quadratic delay determined by the p hugoid frequency.l Information on Wind taken from Elliot Report T. 196, Wind in Relation to Automatic Landing, by C. S. Durst, gives wind distribution between 500 ft. altitude and ground level. Above that altitude the windshear effect is relatively small; its effect and the effect of vgusts would cause only transients, which would be essentially corrected when the I.L.S. limit is reached. Thus the effect at I.L.S. limit of wind above 500 ft. level can be safely ignored.

In view of the above, it is not feasible to use the longitudinal accelerometer of the slaved platform to derive range and range must be computed according to FIGURE from altitude measurement h in connection with angular I.L.S. glide slope deviation (82). Referring to FIG- URE 10, the glide slope range is In the region between I.L.S. glide slope inner limit (say 200 to ft. altitude) to localizer limit (say 50 ft. altitude) no glide slope information is available and z is replaced by zero. The derived range is monitored at the middle marker to reduce error and a circuit for deriving range, monitoring at thev middle marker and for producing I.L.S. distance information will be described below.

The use of the slaved platform does not prevent extension of the use of the system beyond touch-down.

. Error values due to I.L.S. noise are ascertainable, e.g. for a ground speed of 200 ft./sec. to the instant, when the aircraft, nishing its run out, comes to a stop. For example, a deceleration of 0.2 g is assumed, corresponding to a running-out time of 3l sec. and a distance from touchdown to stop of 3100 ft. The errors, (based on an example of time constants which is achieved for all ground speeds by the variable gain system described in section 4) are small. This suggests a possible extension within this invention, of range of the inertial system beyond touch down to nal stop. In nil or low visibility, run-out on a runway involves a risk which a competent system will cope with, and this invention may, finally, cope.

As in the variable gain system the effect of initial transients s reduced to negligible values. In a system based on a high grade gyro stabilized platform the errors Es would represent practically all the errors, and thus extension of range into the ground-borne phase is possible.

In case of the platform slaved to V.G. and D.G. and corrected by a single additional gyro, compass errors in connection with longitudinal deceleration would cause very large cross beam deviation errors (up to 100 ft.) at stop for a compass error of 2 at 0 headwind. In case of windshear, the Windshear errors are of opposite sign and thus partially cancel out the deceleration errors during run out.

To reduce this error there is an arrangement of the invention in which after touch down, the compass error is corrected. The correction procedure is based on the assumption that after touch down, when the aircraft has sufliciently slowed down (say after ca. 5 sec.) the undercarriage takes a larger portion of the weight of the aircraft and can be so adapted as to signal such condition, and in this condition no sideslip on the under-carriage wheels is likely to occur, and the instantaneous direction of the run out path must coincide with aircraft heading. Were it not so, very small external disturbances (gusts) would cause an uncontrollable side slip, just as in the case of skidding motor cars, e.g. due to an ice patch, or other local, immediate, condition.

As to this assumption, sufficient data are available to compute the value of compass error; to subsequently correct the platform position by the computed amount, and also to apply corrective terms tothe deviation and velocity output, in principle completely cancelling the fortuitously induced error.

Additional apparatus requires is: y

(1) Tachometric device on an undercarriage wheel to give rotational speed and thus determine ground speed. This device may be omitted and the parameter estimated as explained hereinafter.

(2) Computing circuit for effecting runway correction as hereinbelow described.

vAccording to FIGURE l1, at the instant of making lthe correcting actions the direction of the run-out track is determined by where y=true cross beam or lateral velocity.

v =longitudinal velocity (ground speed).

`vc=longitudinal velocity (ground speed) atthe instant of erection.

1 5 If y* is the cross beam 0r lateral velocity indicated by the inertia equipment,

1J*=J+ Y1J+Y where:

The velocity error accumulated during the pure inertia 15 period is Some velocity error also accumulates during the monitored period; its effect is however very much smaller, because at the higher altitudes the windshear is smaller and furthermore the error is to a considerable extent corrected by the monitoring feedback; thus as a first approximation represents the whole velocity due to compass error accumulated at the instant of correction.

Aq can be determined as follows:

Substituting /Yp yields r is determined by relative heading, i.e. the difference between compass indication and runway heading; y* is given as inertial lateral cross beam velocity indication; assuming first, that vL and vc are as well known and YIJ=O, the compass error is given by 14.1. lf the platform position is corrected by Ao and a cross beam velocity correction of -A p(vL-vA) is provided, the cross beam velocity error arising from compass error would be, from the instant of correction onwards, eliminated and the resulting distance error at the end of the run-out greatly reduced.

If YU is unknown and generally not zero, an approximation Apc of compass error Aq: can be computed from 14.1 by ignoring YH i.e.

All

and the corrections Acc to platform position and -Aq (vL-vc) to velocity output applied.

If there were no correction, the resultant velocity error from compass and other errors would be:

the second term representing the effect of the velocity correction, and the third term that of platform position 16 by factor v/vL gradually to zero at the end of the run-out, thus greatly reducing distance error.

Assuming that the instant of correction is determined in all cases by the air speed dropping after touch-down to vA=150 ft./sec. and considering first the case where there is no head wind and VL=VAL=200v ft./sec., then Apc (in radians) could be determined from:

where a=0.75 and =0.005.

Assuming limiting values for head wind and wind shear condition: vL=134 ft./sec., v=102 ft./sec., Agoc=is obtained as:

where nt-:0.786, =0.0075.

The value of coefiicients a and would enable the best possible deduction of platform position error due to compass error with the data available, but they would not necessarily have the most favorable effect in respect of other errors. It should be noted that the Variation of the coefiicients in the two extreme conditions is comparatively small. Furthermore, exact determination of the coefficients would involve measurement of ground speed which offers considerable diiiiculties.

It would be preferable as an approximation to replace the variable coefficients a and by absolute constants and also, in order to simplify the system still further, to omit velocity and distance correction and compensate for this by increasing the values for fx and Reasonably good values for reducing run-out error are found to be: a=1.2 and =0t01 ft./sec. y

In the case of otherwise very large errors the correction procedure yields a very large improvement, the resulting error becoming in most cases even smaller than the sum of I.L.S. noise and instrument errors without compass errors.

It should be noted that, assuming a mean value of acceleration during the run-out of say A-0.2 g, the platform position correction could be replaced by an acceleration correction of A pc 0-2 g.

(1) The runway parameters considered, FIG. l2, are:

Runway heading pR. Latitude A. Middle marker glide slope range R11. Distance between localizer and glide slope transmitter Ryz. Glideslope angle 11.

These are set manually by potentiometers. To facilitate setting and avoid errors, it would be convenient to position the potentiometers close together so that a pre-printed card for each runway can be slipped over them showing the potentiometer positions.

cpR is for platform setting.

Cos A cos fpR is required as a correcting torque on the controlling gyro 13 to compensate for the earths rotation.

Ryz is required for computing the localizer range.

Rm for monitoring and adjusting the range at middle marker.

(2) The range unit 46 and I.L.S. distance unit 48 of FIG. 12 are shown in detail in FIG. 13. As mentioned above, Ithe parameters Ryz, Rm and n are set manually by the respective potentiometers 50, 52 and 54. In the normal operating condition (switches 70, 72, 74 and 76 as shown) the servo 56 driven by the amplifier 58 with the feedback path 60 as shown moves the taps 62 tending to zero the voltage sum at junction I through the 

4. IN A LANDING SYSTEM FOR AIRCRAFT COMPRISING, IN COMBINATION: GYRO MEANS HAVING AN OUTPUT Y INDICATIVE OF ACCELERATION OF THE AIRCRAFT IN A DIRECTION LATERAL WITH RESPECT TO A PREDETERMINED GLIDE SLOPE PATH; INTEGRATOR MEANS FOR INTEGRATING THE OUTPUT Y OF SAID GYRO MEANS TO PRODUCE A LATERAL POSITION OUTPUT Y* INDICATIVE OF THE LATERAL POSITION OF THE AIRCRAFT WITH RESPECT TO THE GLIDE SLOPE PATH ACCORDING TO THE GYRO MEANS; I.L.S. RECEIVING MEANS FOR PRODUCING AN OUTPUT SIGNAL Y INDICATIVE OF LATERAL DEVIATION OF THE AIRCRAFT WITH RESPECT TO THE GLIDE SLOPE PATH; COMPARATOR MEANS CONNECTED TO THE OUTPUTS OF SAID I.L.S. RECEIVING MEANS AND SAID INTEGRATOR MEANS AND HAVING AN OUTPUT Y-Y* INDICATIVE OF THE ERROR IN THE OUTPUT OF SAID GYRO MEANS; AUTOPILOT MEANS FOR CONTROLLING THE AIRCRAFT ACCORDING TO SAID OUTPUT Y*; AND FEEDBACK MEANS FOR MODIFYING SAID OUTPUT Y* ACCORDING TO SAID OUTPUT Y-Y*. 